w WAN-0131 Issues when Grounding D.C. Coupled (Capless) Headphone Outputs INTRODUCTION In some situations, the headphone output of a product could be connected to another product, rather than headphones. In this situation it would appear that the “capless” (d.c. coupled) headphone mode, currently provided by the WM9711L, WM9712L, WM8750L, WM8751L and WM8955L, could not be used and large a.c. coupling capacitors would be needed instead. This application note looks at the issue of whether the output would be actually grounded by the connection or not, and therefore if a.c. coupling would be needed. APPLICATION AC-COUPLED HEADPHONE OUTPUT The circuit diagram below shows how to connect a stereo headphone to the headphone output. HPOUTL C1 220uF HPOUTR WM DAC C2 220uF HPGND = 0V Figure 1 Simple Headphone Output Circuit Diagram The DC blocking capacitors C1 and C2 together with the load resistance determine the lower cut-off frequency, fc. Increasing the capacitance lowers fc, improving the bass response. Smaller capacitance values will diminish the bass response. For example, with a 16 Ohm load and C1 = 220µF: fc = 1 / 2π RLC1 = 1 / (2π x 16Ω x 220µF) = 45 Hz DC COUPLED (CAPLESS) HEADPHONE OUTPUT In the interest of saving board space and cost, it may be desirable to eliminate the 220µF DC blocking capacitors, with the added benefit of improved frequency response. This can be achieved by using OUT3 as a headphone pseudo-ground, as shown below. HPOUTL WM DAC HPOUTR OUT3 = AVDD/2 Figure 2 Capless Headphone Output Circuit Diagram WOLFSON MICROELECTRONICS plc www.wolfsonmicro.com June 2003, Rev 1.1 Copyright 2004 Wolfson Microelectronics plc WAN_0131 As the OUT3 pin produces a DC voltage of AVDD/2, there is no DC offset between HPOUTL/HPOUTR and OUT3, and therefore no DC blocking capacitors are required. However, this configuration has some drawbacks: • The power consumption is increased, due to the additional power consumed in the OUT3 output buffer. If the DC coupled output is connected to the line-in of a grounded piece of equipment, then OUT3 becomes short-circuited. Although the thermal cut-out protection will prevent any damage to the IC, the audio signal will not be transmitted properly. • SOLUTIONS HEADPHONE OUTPUT GROUND CASE 1 In the diagram below, the capless output is connected to a hi-fi amplifier, which is grounded. (This is only sometimes true.) In this case the DAC is powered by an isolated supply with no ground, like a typical plug-top PSU. Here it is acceptable to connect the headphone output to the amplifier, as there is no ground connection back to the DAC PSU to cause a short-circuit. HPOUTL HPOUTL HPOUTR HPOUTR AVDD/2 GND AC AC DAC PSU No ground WM DAC GND AMP GND Figure 3 Acceptable Ground Connection 1 HEADPHONE OUTPUT GROUND CASE 2 In the diagram below, the capless output is connected to a mini PC amplifier, which is powered by an isolated supply with no ground, like a typical plug-top PSU. The DAC is inside a PC, which is grounded. In this case it is acceptable to connect the headphone output to the amplifier, as there is no ground connection back to the DAC PSU to cause a short-circuit. HPOUTL HPOUTL HPOUTR HPOUTR AVDD/2 GND AC AC PC PSU WM DAC AMP No ground GND GND Figure 4 Acceptable Ground Connection 2 w June 2003, Rev 1.1 2 WAN_0131 HEADPHONE OUTPUT GROUND CASE 3 In the diagram below, the capless output is connected to an amplifier, which is powered by a grounded supply. The DAC is inside a PC, which is also grounded. In this case it is not acceptable to connect the headphone output to the amplifier as there is a ground connection back to the DAC PSU, which will cause a short-circuit. HPOUTL HPOUTL HPOUTR HPOUTR AVDD/2 GND AC AC PC PSU GND WM DAC GND AMP GND GND Figure 5 Bad Ground Connection 1 HEADPHONE OUTPUT GROUND CASE 4 In the diagram below, the capless output is connected to an amplifier, which is powered by a grounded supply. The DAC is inside a battery-powered CD player, which is grounded because it is powered by a non-isolated car charger. In this case it is not acceptable to connect the headphone output to the amplifier as there is a ground connection back to the DAC PSU, which will cause a short-circuit. HPOUTL HPOUTL HPOUTR HPOUTR AVDD/2 GND DC DC CAR CHARGER GND WM DAC GND CAR AMP GND GND Figure 6 Bad Ground Connection 2 CONCLUSION In most systems where the capless headphone output is used, it is safe to connect it to another piece of powered equipment, instead of headphones. In the rare case where both units are powered by a grounded supply, there will be a short-circuit, which will result in badly distorted sound output. In this case the thermal cut-out protection circuitry will stop any damage to the IC. APPLICATION SUPPORT If you require more information or require technical support please contact Wolfson Microelectronics Applications group through the following channels: Email: Telephone: Fax: Mail: [email protected] +44 (0)131 272 7070 +44 (0)131 272 7001 Applications at the address on last page. or contact your local Wolfson representative. Additional information may be made available from time to time on our web site at http://www.wolfsonmicro.com w June 2003, Rev 1.1 3 WAN_0131 IMPORTANT NOTICE Wolfson Microelectronics plc (WM) reserve the right to make changes to their products or to discontinue any product or service without notice, and advise customers to obtain the latest version of relevant information to verify, before placing orders, that information being relied on is current. All products are sold subject to the WM terms and conditions of sale supplied at the time of order acknowledgement, including those pertaining to warranty, patent infringement, and limitation of liability. WM warrants performance of its products to the specifications applicable at the time of sale in accordance with WM’s standard warranty. Testing and other quality control techniques are utilised to the extent WM deems necessary to support this warranty. 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